1. Field of the Invention
The present invention relates to a copper-based metal polishing solution and a method for manufacturing a semiconductor device.
2. Description of the Related Art
Formation of a wiring layer is included in the process of manufacturing a semiconductor device, and an etch-back technique is employed for eliminating stepped portions from a surface of the wiring layer. The etch-back technique comprises the steps of forming grooves conforming with a pattern of the wiring layer in an insulating film covering a substrate of a semiconductor substrate, depositing a copper film on the insulating film including the trenches, polishing the copper film with a polishing solution, and selectively allowing the copper film to remain within the trenches to form a buried wiring layer.
It was customary in the past to use a polishing solution prepared by dispersing polishing abrasive grains such as a colloidal silica in pure water. In the conventional technique, the polishing solution is supplied into a polishing pad included in a polishing apparatus, and the copper film formed on the substrate surface is polished, with a predetermined load applied to the polishing pad. In the conventional technique, however, a mechanical polishing, which involves the polishing abrasive grains and the polishing pad, is simply applied to the copper film, with the result that the polishing rate was as low as only 10 nm/min.
Other types of polishing solutions for a copper film or a copper alloy film are disclosed in, for example, "J. Electrochem. Soc., Vol. 138, No. 11, 3460 (1991)", "VMIC Conference, ISMIC-101/92/0156 (1992)" and "VMIC Conference, ISMIC-102/93/0205 (1993)". To be more specific, disclosed in these publications are polishing solutions each consisting of a slurry of an amine-based colloidal silica or a slurry containing K.sub.3 Fe(CN).sub.6, K.sub.4 (CN).sub.6, or Co(NO.sub.3).sub.2. However, the polishing solution disclosed in any of these publications gives rise to the difficulty that there is no difference in the etching rate of the copper film between the immersing step and the polishing step. As a result, the copper wiring layer within the trench is further etched with the polishing solution when the wiring layer is brought into contact with the polishing solution after the etch-back step. It follows that the upper surface of the copper wiring layer within the trench is positioned lower than the upper surface of the insulating film. In other words, it is difficult to form the wiring layer flush with the insulating film, leading to an impaired surface smoothness. What should also be noted is that the copper wiring layer buried in this fashion in the insulating film exhibits a resistivity higher than that of the copper wiring layer buried in the insulating film such that the upper surface of the wiring layer is flush with the upper surface of the insulating film.
Japanese Patent Disclosure (Kokai) No. 7-233485 discloses an additional copper-based metal polishing solution prepared by adding at least one kind of an organic acid selected from the group consisting of aminoacetic acid and amidosulfuric acid, and an oxidizing agent to water. Where elemental copper or a copper alloy is immersed in the copper-based metal polishing solution disclosed in this publication, the etching rate of the copper or copper alloy is very low. However, where the copper or copper alloy is subjected to a polishing treatment with the particular polishing solution, the etching rate of the copper or copper alloy is several to scores of times as high as that in the case of the immersion in the polishing solution. To be more specific, if aminoacetic acid included in the polishing solution disclosed in the above-noted publication reacts with hydrated copper, a complex compound soluble in water is formed, as shown below: EQU Cu(H.sub.2 O).sub.4.sup.2+ +2H.sub.2 NCH.sub.2 COOH.fwdarw.Cu(H.sub.2 NCH.sub.2 COOH).sub.2 +4H.sub.2 O+2H.sup.+
It should be noted that copper does not react with a mixture of aminoacetic acid and water. The reaction given above proceeds in a direction denoted by the arrow, if an oxidizing agent, e.g., hydrogen peroxide, is added to the reaction system, resulting in an etching of copper. If, for example, a copper film is immersed in the polishing solution, an oxide film is formed on the surface of the copper film so as to suppress the etching (dissolving) of the copper film. On the other hand, if the copper film is polished with a polishing pad containing the polishing solution, the oxide film formed on the film surface is mechanically removed by the polishing pad so as to expose the pure copper to the surface. It follows that the copper film is rapidly polished chemically by the functions of the aminoacetic acid and hydrogen peroxide contained in the polishing solution. It should be noted in this connection that the copper or copper alloy film tends to be dissolved in the polishing solution to some extent during a short period immediately after the polishing treatment and before formation of an oxide film on the film surface.
The prior art exemplified above, i.e., Japanese Patent Disclosure No. 7-233485, also discloses a method for manufacturing a semiconductor device in which a buried wiring layer made of copper or a copper alloy is formed by an etch-back technique using the copper-based metal polishing solution described above.